This invention concerns a hair treatment composition which is self-limiting in that it cleaves disulfide bonds of keratin to an extent sufficient to impart a desirable ability to set a wave in the hair, then limits further reaction which may degrade the hair.
When a waving lotion is applied to hair, the following chemical reactions may occur: EQU K.sub.1 SSK.sub.2 +RSH.fwdarw.K.sub.1 SSR+K.sub.2 SH I. EQU K.sub.1 SSR+R'SH.fwdarw.K.sub.1 SH+RSSR' II.
As used in these equations, KSSK represents two keratin protein chains (K) joined by a cystine disulfide (SS) bond, and RSH represents a salt of thioglycolic acid. The usual salts are ammonium thioglycolate (ATG) or monoethanolamine thioglycolate (MEATG) as the waving agent. In these equations, KSSR represents a "mixed disulfide" of one keratin protein chain and the waving agent, and the KSH represents a reduced keratin protein chain liberated from its adjacent protein counterpart by the reaction of the waving agent.
It appears that almost irrespective of the pH and the waving agent (ATG or MEATG) used, the first reaction proceeds readily. That is, the initial cleavage of the interprotein disulfide bond and formation of an asymmetrical mixed disulfide occurs at approximately the same rate and to the same extent at pH7.0 as at pH9.2.
In sharp contrast to the relatively facile completion of the first reaction, the cleavage of the mixed disulfide (K.sub.1 SSR) by a second molecule of reducing agent (R'SH) in the second reaction proceeds with more difficulty. The second reaction appears to be much more pH dependent than the first reaction; increasing pH stimulates the second reaction.
These reactions comprise a two-step mechanism in the chemical reduction of hair for a time sufficient to impart the required degree of softening of the fibers so that a wave can be formed. The wave can then be set by reversing the reactions with an oxidant such as hydrogen peroxide or sodium bromate.
The time of processing required to impart a cosmetically acceptable wave has been judged in any of several ways. With many waving systems the "test curl" method is practiced. This can require appreciable skill on the part of the operator. Alternatively, processing may be for a predetermined time, as specified by the instructions from the manufacturer. This timing method attempts to average hair type and other conditions of application so that the wave is effective on most heads in the specified time without causing excess hair damage due to over processing. Again, to achieve desired results with a broad variety of hair types may require a skilled operator.
For such reasons it has been considered desirable to formulate a self-limiting waving system which cleaves keratin disulfides in the first reaction to an extent sufficient to impart a desirable wave, and then effectively limit further reaction which is degrading to the fiber.
Several techniques have been proposed for self-limiting waving reactions. One concept is to rely on mass action to inhibit the two-step reaction from proceeding toward completion. By including a product of the second reaction (RSSR) in the waving solution, it is believed that the position of the equilibrium will not proceed too far toward cleavage and gradual fiber destruction. Such a technique is described in U.S. Pat. No. 4,273,143 by Klemm et al. Such a composition may include the "dithio" form (RSSR) of the waving agent in the waving solution. Such a solution may contain a diammoniumdithiodiglycolic acid salt or a diammonium-diglyceryldithioglycolate.
In reality, true equilibrium is not obtained in the time frame of a salon permanent wave and the dithio form of the waving agent does not appear to limit the reaction. Compositions relying solely on the mass action effect may continue to react with the mixed disulfide with concomitant increasing destruction of the treated hair.
Another means of limiting the action of waving solutions is disclosed in U.S. Pat. No. 4,532,950 by Lang et al. In this technique, just before use, a composition containing the reducing agent (RSH) is mixed with a second composition containing unsaturated aliphatic compounds. The activity of the reducing agent is decreased by gradual adduction of the reducing agent to the unsaturated carbon-carbon bond. This is probably not much different from starting with a lower initial concentration of the waving agent.
It is desirable to provide a different technique for self-limiting the reaction of a waving composition with hair. Such self-limiting action minimizes the potential for hair damage due to over processing. It is desirable that the composition be no more expensive than present compositions and not require special skills on the part of the operator.